Referring to FIG. 1, a prior art massage tub is illustrated. The massage tub 1 is installed with a suction opening 11, a motor 12, a plurality of connecting pipes 13, and a plurality of water outlets 14. In use, the motor 12 is actuated so as to suck water from the suction opening 11. The water flows through the pipes 13 to the water outlets 14 for spraying out. Referring to FIG. 2, another prior art is illustrated. In each massage tub 2, a plurality of water circulation devices 21 are installed. Each water circulation device 21 has a suction opening 22, at least one water outlet 23, and motor 24. The water circulation devices 21 have no conventional pipe, which can be cleaned and maintained easily.
In use of the above mentioned prior art, when the level is higher than the suction opening, the suction opening will not suck water so that the motor rotates idly and thus it is possible that the motor will be burned out.